Embodiments described herein relate to a diesel engine exhaust treatment system, and more particularly, to an exhaust treatment system which utilizes a selective reduction catalyst (SCR) in combination with an SCR-coated diesel particulate filter, where the system achieves reduced backpressure in comparison with a system which utilizes a single SCR-coated filter.
In recent years, environmental regulations in the United States and Europe restricting diesel particulate emissions have necessitated improvements in the removal of particulates from diesel engine emissions. Diesel engine exhaust contains gaseous emissions such as carbon monoxide (CO), unburned hydrocarbons (HC), and nitrogen oxides (NOx), as well as particulate matter.
Diesel engine exhaust treatment systems are currently used to convert these exhaust components to environmentally acceptable compounds and to remove particulates. Such systems typically include the use of a diesel oxidation catalyst (DOC), a selective catalytic reduction catalyst (SCR), and/or a diesel particulate filter (DPF).
Diesel oxidation catalysts are placed in the exhaust gas stream of a diesel engine and typically contain platinum group metals (PGM), base metals, or a combination thereof. These catalysts promote the conversion of CO and HC emissions to carbon dioxide and water.
Selective catalytic reduction catalysts (SCR) are used to convert NOx to N2 and typically comprise a base metal and utilize an ammonia reductant, typically in the form of aqueous urea, which is injected in the exhaust stream downstream from the diesel oxidation catalyst. After water vaporization and urea hydrolysis, the formed ammonia reacts with NOx in the exhaust gas stream on the SCR catalyst to form N2.
A diesel particulate filter (DPF) collects soot or particulate matter from engine exhaust. As soot accumulates on the filter, an increase in pressure drop occurs across the DPF, which requires regeneration of the filter by combustion of the accumulated particulates at elevated temperatures. A precious metal catalyst is also typically coated on the DPF for the removal of CO, HO, and NH3 slip.
A typical diesel engine exhaust treatment system for light and heavy duty applications includes a diesel oxidation catalyst (DOC), selective catalytic reduction catalyst (SCR), and diesel particulate filter (DPF) in the form of three separate units, each positioned downstream from the other in the exhaust stream. However, while such a system is efficient for meeting current emission regulations, it suffers from a number of drawbacks. For example, in some vehicle applications, the available space for packaging all of these components is limited. The use of three separate units results in a large system which produces a high exhaust counter-pressure (backpressure) as the exhaust flows through each unit. In addition, the use of large sized diesel particulate filters including a precious metal catalyst coating makes the system expensive to produce due to the high cost of platinum group metals.
An alternative diesel exhaust treatment system is described in U.S. Patent Application 2005/0031514, which includes a diesel oxidation catalyst positioned upstream from a diesel particulate filter, where the filter includes a selective catalytic reduction catalyst coated thereon such that the diesel particulate filter performs two catalytic functions, i.e., removal of particulates from the exhaust stream and conversion of NOx to N2. The system further includes a separate SCR catalyst or slip oxidation catalyst unit downstream from the filter. While such an integrated system has a smaller volume, it is known that the presence of a DPF generally results in an increase in backpressure in the exhaust system, which adversely affects catalyst performance and fuel economy. As a result, where the SCR is integrated with the DPF, a higher catalyst loading of up to 2 g/in.3 is typically required in order to achieve high NOx removal efficiency and durability. However, such an increased catalyst loading results in an increase in backpressure and lower fuel efficiency.
Accordingly, there is still a need in the art for a diesel engine exhaust system which efficiently achieves conversion of components and removal of particulates in the exhaust gas in an efficient and cost-effective manner, which provides good fuel efficiency, and which reduces backpressure.